Sensing devices employed to determine the amount of a specific gas in a mixture of gasses such as automotive exhaust are presently based on measurements of the electrical resistance of certain ceramic oxides. Such resistance changes directly reflect the amount of specific gas present. Transition metal oxides, particularly TiO.sub.2 are frequently used for such application. The use of such materials for this application is disclosed in particular by U.S. Pat. Nos. 3,886,785 and 3,932,246.
Relatively pure TiO.sub.2 made by solution precipitation techniques has a tetragonal crystalline structure and is called anatase titania. Titania or TiO.sub.2 that is generally employed as a ceramic sensor material for sensing carbon dioxide, carbon monoxide, hydrocarbons, alcohol, fumes, humidity and the like in atmosphere or to determine the carbon dioxide, carbon monoxide, oxygen, nitrous oxide and sulfur dioxide contents of automobile exhausts employ a nontetragonal crystalline titania called rutile titania. In the fabrication of the sensors of U.S. Pat. Nos. 3,886,785 and 3,932,246 anatase titania is converted to rutile titania for this application.
It has now been discovered that anatase titania sensors are particularly useful in the determination of carbon monoxide and hydrogen in gases particularly at elevated temperatures (up to about 850.degree. C.). It has also been discovered that anatase titania in combination with alumina is selective to the determination of hydrogen in a gas mixture and that anatase titania in combination with yttria is selective to the determination of carbon monoxide, particularly when a small amount of a catalyst such as iron or palladium is included.
It is therefore an object of the present invention to provide anatase TiO.sub.2 sensors useful for the determination of hydrogen and carbon monoxide in gases.
It is also an object of the present invention to provide anatase TiO.sub.2 --Al.sub.2 O.sub.3 sensors useful for the selective determination of hydrogen in gases.
It is also an object of the present invention to provide anatase TiO.sub.2 --Y.sub.2 O.sub.3 sensors useful for the selective determination of carbon monoxide in gases.
A further object of the present invention is to provide anatase TiO.sub.2 --Y.sub.2 O.sub.3 sensors containing small amounts of catalysts for the selective determination of carbon monoxide in gases.
An additional object of this invention of to provide anatase-yttria sensors that contain Fe or Pd as catalysts for the selective determination of carbon monoxide in gases.